Effect of copper on tensile properties and grain-refinement of steel and its relation to precipitation behavior

Progress of Ferrous Nano-Metal Project is introduced in the present paper. In the project, maximum use of copper clusters and precipitates is pursued for achieving better strength-ductility balance than that of conventional high-tensile strength steels. Fundamental aspect of clustering and precipitation of Cu in Fe-Cu alloys was studied using Optical Tomographic Atom-Probe (OTAP). It was found that Cu precipitation during aging was enhanced by plastic deformation. The observed Cu precipitation behavior was well related to the age-hardening behavior, that is, aging started at lower temperature and maximum hardness was higher for plastically deformed and aged ferrite. Aging behavior and associated tensile properties were further examined for Fe-C-Mn-Cu martensitic steel. Higher value of tensile strength times elongation was achieved in Fe-C-Mn-Cu steel than Fe-C-Mn steel. Finally, effect of Cu precipitation on grain-refinement of ferrite was studied for Fe-C-Mn-Cu steel. Ferrite grain smaller than 1 μm was obtained in both processes of strain-assisted ferrite transformation from heavily deformed austenite and dynamic recrystallization of heavily deformed ferrite. Ferrite grain size was found to decrease by addition of Cu at the both processes. It was suggested that a simple additivity rule does not hold in terms of the strengthening by grain-refinement and that by precipitation, especially at grain-size range less than 1 μm.